Electron discharge tube



Oct. 23, 1951 J. D. M GEE ELECTRON DISCHARGE TUBE Filed Aug. 19, 1948 lnyen/ar 5y x/mnes 00. McGee Patented Oct. 23, 1951 2,572,644 ELECTRON DISCHARGE TUBE James Dwyer McGee, London, England, assignor to Electric & Musical Industries Limited, Hayes, England, a British company Application August 19, 1948, Serial No. 45,139 In Great Britain August 279, 1947 invention relates to electron discharge tubes of the kind wherein a magnetically focussed electron beam is directed to a target and wherein means are provided for producing an electric field for retarding the electrons in said beam as it approaches said target. An example of such a, tube is the known television pick-up tube in which the electron beam is arranged to scan a photo-electric mosaic which is stabilised at cathodepotential and wherein the electrons are arranged to impinge substantially normally on said mosaic. It is customary to provide the above-mentioned decelerating electric field by means of an annular electrode to which a voltage is applied to produce a suitablepotential gradient along the electron path, that is to say, parallel to the tube axis. However, it is found in practice that in addition to said desired potential gradient there is produced a potential gradient normal to the electron path whereby an accelerating force is exerted on the electrons at right angles to the tube axis, that is to say a radial accelerating force. This force is undesirable since it increases from the centre to the edges of the scanning raster and, in conjunction with the axial magnetic focussing field, produces the effect known as spiral distortion. The object of the invention is to provide an electron discharge tube of the kind first referred to above in which the above-mentioned efiect is reduced or avoided.

According to the invention the magnetic field which is employed for focussing the beam is arranged to be divergent to a suitable degree, in the direction of the motion of the electrons as they approach the target, in the region of said decelerating electric field whereby substantially to compensate for the above-mentioned radial force.

The efi'ect of causing the magnetic focussing field to diverge as stated is to introduce a radial component of the magnetic field, said component exerting on the electrons, by virtue of their axial velocity, a force which is substantially oppositely directed to that produced by the radial electric field and the axial magnetic field.

In the application of the invention to a television pickup tube of the type described, the desired divergence of the magnetic field may be effected by the provision of a short auxiliary coil around the focussing coil and by passing a cur- 7 rent through it in a direction such as to produce a magnetic field which is opposed to that produced by the focussing coil. Another way of producing said divergence is to form the focussing 6 Claims. (01. 313-84) coil in two axially spaced parts. If desired the spacing of said parts may be adjustable whereby to enable optimum distortion of the magnetic field to be obtained. Still another way of producing said divergence is to position the tube near the end of the focussing coil so that the a divergent magnetic field at the end of said solenoid has a component which counteracts the eiiect of "the above-mentioned radial electric field.

In order that the said invention may be clearly understood and readily carried into effect, the same will now be more fully described with reference to the accompanying drawing, the three figures of which show in longitudinal section three electron discharge tube arrangements according to the invention. In the various figures the cathode 2 from which the electron beam employed for scannin the target emanates, said cathode being shown grounded. The tube comprises an envelope which includes a neck 3 and a bulb 4 the latter having a transparent end wall '5, through which an optical image for transmission is projected. On the inner surface of neck 3 and part of bulb 4 is provided a conducting coating 6 to which in the operation of the tube a suitable potential (e. g. +250 volts) is applied for the purpose of accelerating the beam,

as will be understood. 0n the inner wall of bulb 4 adjacent screen I and spaced from coating 6 to output terminal l3. ;:arranged to be deflected under the action of is provided an annular electrode formed by a conducting coating 1 to which a relatively low positive potential (e. g. +5 volts) is applied and which serves to retard the electrons as they approach the target I. Between target I and end wall 5 is provided a transparent signal plate 8, shown connected to ground via resistor l4 and The electron beam is f 'scanning coils indicated at 9, and to be focussed on target I by means of a focussing coll consti- L- tuted by a long solenoid l0 which projects beyond 'end wall 5 so that the magnetic field produced by the solenoid is substantially uniform and parallel to the tube axis over the entire length of the electron path between gun 2 and target I.

As stated above, in addition to the potential gradient produced by electrode 7 in the direction of the tube axis there is also produced a potential gradient normal to the electron path which in conjunction with the axial magnetic field produced by solenoid. 10 gives rise. to spiral distor: tion. Inord'er to. overcome or reduce this defect an auxiliary coil formed by a short coil H is provided which surrounds solenoid which is located adjacent the target I, and a current is passed through coil ll i'ILa. direction; such as to provide a magnetic field which is op- ID and.

posed to the magnetic field producediby solenoidji III. caused to diverge in the region of the retarding electric field produced by electrode l whereby: there is imparted to the magnetic field-in this Thereby the resultant magnetic "field is.

of said decelerating electrostatic field on the movement of the electrons.

2. An electron discharge tube arrangement acproduced by a focussing coil disposed to provide 7 thedivergent, part of its field in the region of said electric field.

A=n electron. discharge tube arrangement comprising an electron discharge tube having an region a radial component which by virtue; of the V axial velocity of the electrons exerts on them a force which is substantially oppositely directed to-jthejforce. produced by the radiallyelectricfield andithe axially magneticv field. 'By suitably rel'ating the magneticfields produced by solenoid [EL and coil l I. to theradial force which produces spiral. distortion can be substantially compensated.) p

In the arrangement shown inFi'gure 2, the tube construction is the same as in Figure 1.. In this case, however, the focussing solenoid IO is formed in two parts which are separated by a gap. l'2 whereby-'bythe desired divergence of: the magfield-rin theregionof' electrode 1. is prodilcedf. v In the arrangementshown in Figure 3' the coil I'll; instead ofv projecting beyond end. wall 5 so far that] the. magnetic field produced is substantially uniform over the electron path, projects onlyto.-such.a distance. that the magnetic field diverges, as indicatedby the lines of. force l5, to

The fiollowing references are of record in Q a suitable extent in the region of electrode 1' so .as. to compensate for the. radial. force which produces. spiral distortion.

Whatlclaim is: f

, l. An. electron discharge tube. arrangement comprising an electron. discharge tube having an k-aesmosa V electron gun for producing an electron beam, a

target, means for deflecting said beam to scan said target, a retarding electrode disposed adiacent said target, an accelerating electrode disposed between said gun'and said retarding electrode,. means for applying potentials to said acc'elerating and retarding electrodes proportioned to, accelerate and then retardthe electronsin their. passage towards said target, wherebyan' electrostatic'field is produced tending to produce" spiral di'stortionand means for producing a, magnetic field divergent. in the direction of motion of the, eleetronsand in the region of said electrostatic field whereby the effector said electrostaticfield is at least partly overcome.- An electron discharge tube arrangement ac"-- cording to claim 5 wherein said divergent field is produced by a relatively long beam focussing coil in conjunction withv a relatively short auxiliary I coil. 7

v JAMES DWYER MCGEE.

REFERENCES CITED}.

file. of this patent: I

. UNITED STATES PATENTS 7 Date Law Jan. 25,19 9 

